adding jest test setup

Summary:
Adds a test runner for jest test and adds three simple test cases:
- render the app
- start a server
- client connecting to the app

Test can be run using `yarn test`.

To make the test runner work, some changes needed to be made:
- remove the export of `init()` from `'flipper'`, because it was a cyclic dependency
- updating Button.js to the new ref-API

Reviewed By: jknoxville

Differential Revision: D10027078

fbshipit-source-id: 49107b0dd4dec666b92ecd841422fe7e6b3a7756

src/reducers/index.js

@@ -30,7 +30,7 @@ export type Store = ReduxStore<
     connections: DevicesState,
     pluginStates: PluginsState,
   },
-  ApplicationAction | DevicesAction | PluginsAction,
+  ApplicationAction | DevicesAction | PluginsAction | {|type: 'INIT'|},
 >;
 
 export default combineReducers({